1. Field of the Invention
This invention generally relates to microelectronic device manufacturing and, more specifically, to methods and systems for measuring a voltage of a microelectronic topography.
2. Description of the Related Art
Inaccurate analysis of one or more parameters within a microelectronic device, such as a transistor, may hinder or prohibit the function of the device, leading to a reduction in production efficiency and device quality. The characterization of thin films is especially important, since the effectiveness and reliability of thin films play an important, central role in the operation of a microelectronic device. The term “thin film” is commonly used within the microelectronic industry when referring to layers deposited upon a wafer during the fabrication of a microelectronic device. Thin film materials may include, but are not limited to, metallic, semiconductor, and dielectric materials or a combination of such materials. Often, thin films are doped with impurities to heighten the effectiveness of the material used. In order for a thin film to be effective, it must conform to strict electrical, chemical, and structural requirements. Therefore, thin films must be accurately analyzed in order to meet a microelectronic device's functionality requirements. In addition, as production volumes and efforts to improve process control increase in the integrated circuit fabrication industry, the ability to accurately characterize microelectronic processes and the materials associated with such processes in a timely manner becomes more critical.
A thin film may be characterized by a number of different properties. For example, a thin film may be characterized by its composition, physical thickness, and/or electrical properties, to name just a few. Consequently, a number of different techniques may be used to characterize a thin film. For example, electrical test techniques may provide electrical capacitance, electrical thickness, and electrical conductivity information about thin films. It is a significant advantage to be able to directly measure electrical properties of a material since the end-usage of microelectronic products is electrical in nature. In some cases, non-contacting systems may be used for such measurements, providing electrical information without having an electrode physically contact a thin film. In this manner, damage to the microelectronic topography may be prevented.
Non-contact techniques typically use an ion generation source such as a corona to source, and a non-contacting voltage measurement sensor such as a Kelvin probe, a Monroe probe, or an atomic force microscope probe, to determine electrical properties of the films. Examples of such systems are illustrated and/or described in U.S. Pat. No. 5,485,091 to Verkuil, U.S. Pat. No. 5,594,247 to Verkuil et al., U.S. Pat. No. 6,097,196 to Verkuil et al., and U.S. Pat. No. 6,202,029 B1 to Verkuil et al., which are incorporated by reference as if fully set forth herein. In order to provide highly accurate results, it is generally advantageous to control corona charge deposition to a high level of precision and uniformity. Conventional techniques that are able to deposit uniform charges, however, usually suffer from a low deposition rate. As such, there is typically a trade-off between uniformity and deposition rate when using non-contact electrical testing techniques.
Another difficulty with non-contacting testing systems is that the measured voltage includes contributions not just from the surface to be measured, but also from the work function of the surface being measured and the work function of the sensor used for the measurement. In general, any non-contacting voltage measurement between a probe and a sensor will result in a measured voltage of:Vmeasured=Vprobe—surface+φms where Vmeasured is the measured voltage, Vprobe —surface is the desired voltage to be measured, and φms is the work function difference between the probe and measured surface. In ambient conditions, φms is difficult to determine and is a function of not only the material properties of the sensor and measured surface, but of ambient conditions such as temperature, humidity, water layers on the measured surface, partial pressures of various trace gases in the air, and air-borne molecular contamination. φms is also known to drift over extended periods of time. For example, practical measurements of the stability of φms show that it is difficult to prevent drifts on the order of greater than about 10 mV over timescales of hours or several hundred mV over a longer time period. Therefore, uncertainty in φms adds uncertainty to the measured voltage.
The work function of a sensor may have a particularly strong influence on such measurement variations since the sensor is used for multiple measurements. The exposure of the substrates to be measured, on the other hand, is limited and is usually consistent since they are measured at the same point in the fabrication process. In any case, in an effort to overcome the problems associated with ambient environments, non-contact electrical testing techniques are sometimes conducted in an ultra-high vacuum environment. Unfortunately, however, an ultra-high vacuum environment is typically costly and difficult to maintain, making the use of such an environment impractical for many applications.
As such, it would be advantageous to develop a system and a method for calibrating the work function of a non-contact voltage sensor such that electrical properties of a thin film may be measured in an ambient environment. In addition, it would be advantageous to develop a corona discharge gun that deposits corona in an efficient and uniform manner.